This invention relates to a retentive and removable trial bearing insert for use with a total hip replacement joint which includes a dual mobility prosthetic cup and a total hip replacement joint including such a trial bearing insert.
Bipolar cups are well known and have the spherical bearing head of the stem rotatable within the inner surface of an outer bearing having an outer surface which, in turn, rotatably engages the natural acetabulum. Such are shown in U.S. Pat. Nos. 4,798,610 and 5,062,853.
Dual mobility cups, sometimes referred to as tripolar cups, include a part-spherical bearing insert with a part-spherical bearing surface which engages a part-spherical inner bearing head on a femoral stem in a bipolar cup. In addition the bearing insert has an outer bearing surface which is freely movable in an outer cup, usually metallic, which may be fixed to the acetabulum˜ Prosthetic cups of this kind are used to provide a greater range of relative movement between the ball head on the femoral stem and the outer cup of the prosthesis.
Dual mobility cups of this type are well known and a dual mobility cup construction is described, for example, in U.S. Patent Publication No. 2005/0060040.
In cups of this type the ball head is “constrained” in that it can be maintained in place in the insert by means of a ring or by a deformation of the insert itself when inserting the head due to the inner diameter of an insert being smaller than the diameter of the head. This type of system is sometimes referred to as an anti-dislocation system because it is harder to dislocate a big head than a smaller one. As mentioned above, this system also offers a wide range of motion because of the rotation of the insert.
When a surgeon puts a total hip replacement joint in place in the patient the ball head which is mounted on a femoral stem is put in place in the femur and the definitive cup is installed in the patient's hip but the surgeon needs trial reduction to check the leg length and the stability of the joint. The surgeon usually prefers to use trial instruments, that is a trial ball head and a trial bearing insert. The problem with dual mobility cups is that if a trial bearing insert is employed there appears to be only the type of device shown in FR 2 785 525 to make inserting and removal of the ball head easy but this requires a specially shaped ball head, and is intended for use with an outer cup not a dual mobility cup.
The present invention is intended to provide a construction of a retentive and removable trial head and bearing insert for use in bipolar and dual mobility cups.